1. Field of the Invention
This invention relates to an image forming device and a computer-readable recording medium.
2. Description of the Related Art
An image forming device of electrophoto-graphic type according to the related art is known. The image forming device of this type includes a charging part, an exposure part, a developing part, a transfer part, and a cleaning part, which are disposed around the periphery of a cylindrical photoconductor drum. During operation of the image forming device, a toner image is formed on the surface of the photoconductor drum when the photoconductor drum is rotated, and the toner image is transferred from the photoconductor drum to a printing medium, such as a copy sheet.
In the image forming device of this type, a toner charging process is performed with the developing part for frictional charging of a toner in the developing part. The amount of charge of the toner generated by performing the toner charging process varies depending on the temperature and the humidity of the atmosphere in which the toner exists.
There is almost no influence on the amount of charge of the toner by the atmosphere during a normal image formation operation (or a printing operation). However, if the image forming device restarts a printing operation after an end of a standby state being continued over a certain period of time, a significant influence on the amount of charge of the toner by the atmosphere may appear during an image formation operation immediately after the restart due to the humidity change.
Normally, a developing part for supplying a toner to a photoconductor drum is arranged to store the toner in a closed space in order to prevent scattering of the toner. FIG. 1 is a diagram for explaining a movement of toner in a vicinity of a developing device. As illustrated in FIG. 1, a toner 6, a developing roller 3, and a supplying roller 4 are accommodated in a toner container part 2a of a developing device 2. An opening of the developing device 2 is substantially closed by a developing blade 5, the developing roller 3, and a toner return part 2b that constitutes a part of a side wall of the developing device 2.
When the image forming device is in an image formation operation and in a standby state, the gap between the developing blade 5 and the developing roller 3 and the gap between the developing roller 3 and the toner return part 2b are in a closed state with the toner 6.
During the image formation operation, a part of a toner 6b that is delivered from the developing device 2 by the developing roller 3 adheres to a surface 1a of a photoconductor drum 1 to form a toner image (an image formation toner 6c) on the surface 1a of the photoconductor drum 1, and the remainder of the toner 6b is returned through the toner return part 2b to the inside of the developing device 2 (a return toner 6d).
However, if the image forming device is set in a standby state of image formation, the toner 6b outside the developing device 2 remains adhering to the developing roller 3 and is not returned back to the inside of the developing device 2 which is formed as the closed space. As long as the standby state continues, the toner 6b outside the developing device 2 is easily influenced by a change of the environmental temperature or humidity outside the developing device 2.
Generally, during an image formation operation of the image forming device, the portion in the vicinity of the photoconductor drum is subjected to a high-temperature, low-humidity condition (the relative humidity is lowered by a temperature rise). During a standby state of image formation, the high-temperature, low-humidity condition of the portion is gradually changed to be equal to the external environmental temperature and humidity. The toner 6a inside the developing device 2 is not easily influenced by a change of the external environment because it is stored in the closed space of the developing device 2. In contrast, the toner 6b outside the developing device 2 is easily influenced by a change of the environmental temperature and humidity during the standby state, which results in a deterioration of the toner charging performance.
The formation of a toner image on the photoconductor drum 1 during image formation operation is performed using the action of an electric charge of the toner, and a deterioration of the toner charging performance directly affects the image formation state on the photoconductor drum, namely, affecting the image quality. Subsequently, at a next time of image formation, a toner image is formed on the photoconductor drum 1 by using the toner 6b outside the developing device (which has been affected by the surrounding environment during the standby state) and the toner 6a inside the developing device (which is not affected in the closed space). In such a case, belt-like image density irregularities may occur in the toner image due to the difference in the toner charging performance between the toner 6a and the toner 6b. 
Conventionally, several approaches to eliminating the problem of image density irregularities due to the change of the toner charging performance have been proposed.
For example, Japanese Laid-Open Patent Publication No. 2004-109980 discloses an image forming device which is arranged to perform a rotating operation to rotate the developing roller each time the image formation stopping time reaches a given period, in order to optimize the image density controlling factors.
Japanese Laid-Open Patent Publication No. 2007-065581 discloses a developing device in which an environmental sensor is disposed in a developing part of each color. This developing device is arranged to optimize the toner charging state in accordance with changes of the humidity of the developing part detected by the sensor.
Japanese Laid-Open Patent Publication No. 2004-264647 discloses an image forming device in which a sensor is disposed in the vicinity of a photoconductor drum to detect a change of the environmental humidity during a standby state of the image forming device. When the humidity change detected by the sensor is large, the photoconductor drum is rotated by a given amount of rotation in order to prevent the image density irregularities which easily take place immediately when the standby state is switched to an active state of image formation operation.
In the image forming device of Japanese Laid-Open Patent Publication No. 2004-109980, the rotating operation to rotate the developing roller is performed at intervals of a fixed time during a standby state, and the influence by the outside environment on the toner adhering to the developing roller can be eliminated. However, in this image forming device, the power consumption during the standby state is increased by the rotating operation of the developing roller performed during the standby state.
In the image forming device of Japanese Laid-Open Patent Publication No. 2007-065581, the change of the environmental state of the toner inside the developing device is detected, but the change of the environmental state of the toner adhering to the portion exposed to the outside of the developing device cannot be detected. It is difficult for this image forming device to eliminate the problem of image density irregularities which easily take place at a time of restarting of an image formation operation immediately after an end of a standby state.
In the image forming device of Japanese Laid-Open Patent Publication No. 2004-264647, the sensor has to be disposed in the vicinity of the photoconductor drum to detect a change of the humidity in the vicinity of the photoconductor drum, which increases the manufacture cost and may put the restrictions on the layout of the image forming device.